In the formation of three-dimensional integrated circuits, dies are often bonded on semiconductor wafers. The bonding process typically include selecting known-good-dies (top dies), and bonding the top dies to the bottom chips in a bottom wafer using flip chip bonding. Each of the bottom chips may be bonded to one or more top die. After the bonding, underfill is dispensed into the space between the top dies and the bottom chips, and a molding compound is molded onto the top dies and the bottom wafer. After the molding of the molding compound, the package may have a warpage due to the contraction of the molding compound. Accordingly, stresses may be generated and applied to the bottom wafer and the overlying top dies.
The situation is further worsened after the backside grinding process, during which the silicon substrate in the bottom wafer is grinded, and hence the thickness of the bottom wafer is significantly reduced. The warpage is accordingly more severe. The warpage may cause poor adhesion between the layers in the resulting package, poor resistance to moisture, bump cracking, and the like. As a result, reliability issues are more likely to occur, which reliability issues may be identified in thermal cycle tests, drop tests, bend tests, and the like.